GIFT  Of 


No.  1663 


DESCRIPTION 


OF  THE 


PRATT  RANGE  BOARD 


MODEL  OF  1905 


MECHANICAL  FEATURES  AND  RULES  GOVERNING 

ITS  CARE  AND  PRESERVATION 

IN  SERVICE 


(SIX  PLATES) 


MAY  20,  1908 
REVISED  DECEMBER  30,  1914 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 

1917 


No.  1663 

DESCRIPTION 

OF  THE 

PRATT  RANGE  BOARD 

MODEL  OF  1905 


MECHANICAL  FEATURES  AND  RULES  GOVERNING 

ITS  CARE  A1MD  PRESERVATION 

IN  SERVICE 


(SIX  PLATES) 


MAY  20,  1908 
REVISED  DECEMBER  30,  1914 


WASHINGTON 

GOVERNMENT  PRINTING  OFFICE 

1917 


C"'^ 


0 


War  Department, 
Office  of  the  Chief  of  Ordnance, 

Washington,  December  30,  1914. 
This  Manual  is  published  for  the  information  and  government  of  the  Regular  Army 
and  Organized  Militia  of  the  United  States. 
By  order  of  the  Secretary  of  War: 

William  Crozier, 
Brigadier  Genera  I ,  Chief  of  Ordn  ance . 

(3) 


365242 


Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

IVIicrosoft  Corporation 


http://www.archive.org/details/descriptionofpraOOunitrich 


DESCRIPTION   OF   THE   PRATT   RANGE   BOARD. 

MODEL  OF   1905. 


GENERAL  DESCRIPTION. 

1.  The  Pratt  Range  Board  is  a  mechanical  computing  device  used 
in  determining  the  corrected  range  to  be  transmitted  to  the  gun 
emplacements.  It  consists  of  a  box,  chart  board,  canvas  chart 
mount,  correction  chart,  curve-indicator  system,  correction  ruler, 
chain  for  ruler  and  counterweight,  chain  sprockets,  and  counter- 
weight. 

2.  The  chart  board  is  secured  within  the  box  by  means  of  the  upper 
clips  and  lower  clip.  The  canvas  chart  mount  is  stretched  in  position 
by  the  clamps,  nut  for  clamps,  and  washer  for  clamp.  This  clamping 
device  is  secure  to  the  pipes  for  canvas,  which  in  turn  retain  the  rods 
for  canvas.  The  rods  are  sewed  to  the  margin  of  the  canvas  chart 
mount.  The  canvas  chart  mount  should  be  stretched  in  position 
before  pasting  the  correction  chart  thereto.  The  correction  chart  is 
so  located  that  its  range  scale  is  perpendicular  to  the  bottom  end  of 
the  chart  board.  The  chart  for  the  1 -pounder  subcaliber  gun  is 
mounted  on  the  reverse  side  of  the  canvas  chart  mount. 

3.  The  correction  chart  is  provided  with  four  sets  of  correction 
curves,  designated  ^^ atmosphere,''  ^'velocity,"  ^Hide,"  and  '^wind.'' 
It  has  a  vertical  range  scale  on  each  edge  of  the  chart  and  horizontal 
range  lines  and  data  for  accuracy  tests.  The  curves  reading  from  left 
to  right  are  drawn  to  give  the  range  correction  for  every  2  per  cent 
variation  in  the  density  of  atmosphere,  for  every  10-foot-second  of 
velocity,  for  every  5  feet  of  tide,  and  for  every  10-mile  range  compo- 
nent of  wind.  The  red  line  of  the  chart  is  a  line  of  no  correction  and  is 
called  the  normal.  To  avoid  liability  of  error,  reference  numbers  are 
used  for  atmosphere,  velocity,  and  wind,  instead  of  two  sets  of  num- 
bers with  the  plus  and  minus  signs.  The  horizontal  scale  of  the  chart 
is  200  yards  to  the  inch. 

4.  The  curve  indicator  system  consists  of  the  bar  for  curve  indi- 
cators, curve  indicators,  clamping  screw  for  curve  indicator,  support 
for  curve-indicator  bar,  and  chain  for  clamping  screw  for  curve-indi- 
cator bar.  The  bar  for  curve  indicators  is  the  main  part  on  which  the 
curve  indicators  are  used  to  indicate  the  correction  curve  to  be  used 
in  each  set.  The  bar  for  curve  indicators  is  attached  to  the  box  near 
its  top  and  is  supported  at  the  left  end  by  the  clamping  screw  for 
curve-indicator  bar  and  the  support  for  curve-indicator  bar,  and  on 
the  right  end  by  the  clamping  screw  for  curve-indicator  bar  and  the 

(5) 


bearing  strip  for  ruler.  The  three  tapped  holes  in  the  bearing  strip 
for  ruler  allow  for  the  three  different  settings  of  the  bar  for  curve  indi- 
cators. The  clamping  screw  for  curve-indicator  bar  is  fastened  to 
the  chain  for  clamping  screw  for  curve-indicator  bar,  which  in  turn 
is  secured  to  the  top  of  the  box  by  a  brass  screw  eye. 


CORRECTION  RULER. 


5.  The  correction  ruler  consists  principally  of  the  body,  stationary 
box,  movable  bar,  movable  scale,  four  movable  clamping  devices  with 
pointers,  a  gear  mechanism,  reading  glass  holder  support  slide  with 
cross  wire,  and  reading  glass,  guide  plate  (right)  and  guide  plate  (left) 
with  ruler  clamps  and  ruler  clamp  screws,  and  two  range  pointers. 

6.  The  body  is  the  main  part  to  which  the  other  pieces  are  attached. 
The  scale  engraved  on  the  body  is  graduated  400  yards  to  the  inch. 
The  stationary  bar  is  secured  to  the  upper  edge  of  the  body  by  seven 
0.177  by  0.5  inch  standard  filister  head  bronze  screws  and  acts  as  one 
of  the  guides  for  the  movable  clamping  device  with  pointers.  The 
movable  bar  is  so  designed  that  when  the  knob  is  turned  the  gear 
mechanism  will  move  the  bar  twice  the  distance  of  the  movable 
scale.  The  0.099  by  0.187(i%)  inch  standard  filister  head  steel  screw 
is  located  on  the  top  and  at  each  end  of  the  movable  bar  and  forms  a 
stop  for  the  movable  clamp.  The  movable  bar  also  acts  as  the  other 
guide  for  the  movable  clamping  device  with  pointers  and  consists 
principally  of  the  pointer,  movable  clamp,  eccentric,  stop  screw,  wing 
nut,  clamp  spring,  and  taper  pin. 

7.  The  movable  clamping  device  with  pointers  is  so  designed  that 
each  is  clamped,  independently  of  the  others,  either  to  the  stationary 
bar  or  the  movable  bar,  or  to  both  bars  simultaneously.  When 
clamped  to  both  bars,  the  entire  mechanism  is  locked.  The  provision 
for  locking  to  both  bars  serves  two  purposes — first,  to  provide  against 
shifting  the  pointer  when  passing  from  one  clamped  position  to  the 
other;  and,  second,  to  prevent  the  mechanism  from  getting  out  of 
adjustment.  The  pointer  is  secured  to  the  upper  edge  of  the  movable 
clamp  by  two  0.112  by  0.125  inch  standard  roundhead  bronze  screws. 
One  pointer  is  used  by  each  set  of  correction  curves.  The  letter  '^S" 
on  the  right,  ''L"  in  the  center,  and  ^^M"  on  the  left  are  engraved  on 
the  plan  and  upper  view  of  the  movable  clamp.  The  wing  nut  is 
secured  to  the  eccentric  by  a  0.093  (A)  bj  0.56  inch  taper  pin.  The 
movable  scale,  which  sHdes  in  a  dovetail  slot  in  the  body,  is  moved  by 
the  gear  mechanism  when  the  knob  is  turned.  The  graduated  scale 
is  400  yards  to  the  inch. 

8.  The  gear  mechanism  consists  principally  of  the  gear  bracket,  gear 
cover,  knob,  main  gear,  movable  bar  gear,  gear  and  pinion,  and  idler 
gear.     The  gear  bracket  is  secured  to  the  lower  edge  of  the  body  by 


four  0.177  by  0.75  inch  standard  filister  head  bronze  screws  and  retains 
the  main  gear  and  the  idler  gear.  The  gear  cover  retains  the  gears 
in  position  and  is  secured  to  the  bottom  of  the  body  by  seven  0.112 
by  0.25  inch  standard  countersunk  head  bronze  screws.  The  knob  is 
fastened  to  the  main  gear,  which  meshes  with  the  idler  gear,  which 
in  turn  rotates  the  gear  and  pinion  and  movable-bar  gear.  The  pin- 
ion is  riveted  to  the  gear  and  pinion  and  meshes  with  the  teeth  of  the 
rack  on  movable  scale.  The  movable-bar  gear  moves  the  bar  when 
rotated.  The  knob  should  never  be  turned  except  when  adjusting, 
while  all  four  movable  clamping  devices  with  pointers  are  clamped 
to  the  stationary  bar. 

9.  The  reading-glass  holder  support  slide  is  so  designed  as  to  slide 
in  a  T-shaped  slot  in  the  body.  The  cross  wire  is  secured  to  the  left 
side  of  the  reading-glass  holder  support  shde  by  the  cross-wire  cUps 
and  cross-wire  screws.  The  reading  glass  is  retained  in  the  reading- 
glass  holder  by  the  reading-glass  retaining  screw.  The  reading  glass 
magnifies  the  graduations  on  the  body  and  the  movable  scale.  The 
reading-glass  holder  hinge  screw  forms  a  pivot  for  the  reading-glass 
holder  in  the  reading-glass  holder  support  slide.  The  0.099  by  0.16 
inch  headless  steel  screw  in  the  slot  at  the  right-hand  end  of  the  body 
prevents  the  reading-glass  holder  support  shde  from  coming  out. 

10.  The  guide  plate  (right)  and  guide  plate  (left)  are  secured  to 
the  bottom  and  their  respective  ends  of  the  body  by  four  0.216  by 
0.375  inch  standard  filister  head  bronze  screws.  The  ruler  clamps 
enable  the  correction  ruler  to  be  clamped  in  any  desired  position  to 
the  bearing  strips  for  ruler  with  the  aid  of  the  ruler  clamp  screws. 

1 1 .  The  range  pointers  are  secured  to  the  bottom  of  the  body,  one 
at  each  end,  by  tw'o  0.112  by  0.25  inch  countersunk  head  bronze 
screws,  and  are  used  for  indicating  the  range  of  the  correcting  chart. 

12.  The  correction  ruler  is  maintained  in  any  desired  position  in 
front  of  the  correction  chart  by  the  chains  for  ruler  and  counterweight. 
It  should  be  set  parallel  with  the  horizontal  lines  on  the  correcting 
chart.  One  end  of  each  chain  for  ruler  and  counterweight  is  attached 
to  the  counterweight  by  means  of  a  brass  screw  eye.  Both  chains 
for  ruler  and  counterweight  pass  up  through  the  middle  of  the  top 
of  the  box,  separate,  and  each  passes  over  two  chain  sprockets, 
thence  down  through  the  top  of  the  box,  and  each  is  attached  at 
its  other  end  to  an  end  of  the  correction  ruler.  The  chain  for  ruler 
and  counterweight  is  attached  to  the  right  end  of  the  correction 
ruler  by  means  of  the  chain  screw,  and  to  the  left  end  by  means 
of  the  chain-adjusting  screw  and  cap.  The  cap  is  secured  to  the 
chain-adjusting  screw  by  a  0.063  by  0.31  inch  bronze  pin.  The 
chain  sprockets  rotate  on  the  axis  for  chain  sprockets,  which  in  turn 
are  secured  in  the  sprocket  stand  (right)  and  sprocket  stand  (left). 


ADJUSTMENTS. 


13.  There  are  two  adjustments:  First,  vertical  adjustment. — ^^With 
the  chain-adjusting  screw  on  one  of  the  chains  for  ruler  and  counter- 
weight at  the  left  end  of  the  correction  ruler,  set  the  two  range  pointers 
at  the  same  range  on  the  vertical  scales  on  the  correction  chart. 

14.  Second,  horizontal  adjustment. — Clamp  the  movable  clamping 
device  with  pointers  to  the  movable  bar.  Turn  the  knob  of  the  gear 
mechanism  until  the  pointer  of  the  movable  clamping  device  is  exactly 
opposite  the  normal  of  its  set  of  correction  curves.  Clamp  this  mov- 
able clamping  device  with  pointers  to  the  stationary  bar.  In  like 
manner  set  each  of  the  other  pointers  of  the  movable  clamping  devices 
to  the  normal  of  its  correction  curve  and  clamp  to  the  stationary  bar. 
When  all  four  pointers  of  the  movable  clamping  devices  are  at  their 
normals  and  clamped  to  the  stationary  bar,  turn  the  knob  until  the 
reading-glass  holder  support  slide  with  cross  wire  and  reading  glass 
indicates  the  same  range  on  both  the  scale  graduated  on  the  body 
and  the  movable  scale.  Clamp  one  outside  movable  clamping  device 
only  to  both  movable  bar  and  stationary  bar,  locking  the  system. 

OPERATION. 

15.  Set  each  curve  indicator  at  its  proper  correction  curve.  Set 
the  cross  wire  with  the  aid  of  the  reading  glass  at  the  range  to  the  set- 
forward  point  on  the  scale  graduated  on  the  body.  Set  the  range 
pointers  of  the  correction  ruler  at  the.  range  of  the  set-forward  point 
on  both  vertical  range  scales  of  the  correction  chart.  Clamp  the  mov- 
able clamping  device  with  pointer  to  the  movable  bar,  and  by  means 
of  the  knob  set  the  pointer  at  the  correction  curve  indicated  by  the 
curve  indicator.  Then  clamp  the  stationary  bar.  Proceed  in  like 
manner  with  the  other  movable  clamping  devices  with  pointers  in 
succession,  locking  the  mechanism  with  last  outside  movable  clamping 
device  with  pointer.  The  range  on  the  movable  scale  at  the  cross 
wire  under  the  reading  glass  is  the  corrected  range.  The  range-board 
operator  transmits  this  corrected  range  to  the  gun  emplacements. 

Note. — In  operating  it  is  not  necessary  to  return  any  pointer  to  the 
normal  of  its  set  of  correction  curves.  It  is  necessary  to  set  only  the 
correction  ruler  and  range  pointers  at  the  range  for  the  set-forward 
point  and  move  each  pointer  directly  to  its  proper  correction  curve. 
The  correction  curve  is  frequently  imaginary  and  lies  between  two  of 
the  plotted  curves,  in  which  case  the  pointer  is  set  on  the  imaginary 
curve,  as  indicated  by  the  curve  indicator. 

TEST  OF  ACCURACY. 

16.  There  are  two  tests  for  accuracy,  viz:  For  mechanical  accu- 
racy— Use  each  set  of  test  points  marked  on  the  correction  curves  and 
on  the  same  range  line  and  determine  the  corrected  range.     The  true 


corrected  range  corresponding  to  each  set  is  shown  in  brackets  on  the 
margin.  Repeat  several  times  for  each  set  and  record  the  difference 
of  each  determined  range  from  the  true  range.  The  mean  of  these 
differences  is  the  mechanical  error.  An  officer  should  conduct  this 
test,  exercising  the  utmost  care  in  operating  the  board. 

For  accuracy  of  the  range-board  operator. — Direct  method:  The 
range-board  operator  uses  the  test  points  with  the  range  pointers. 

Reverse  method:  After  a  series  of  corrected  ranges  have  been 
determined  by  the  range-board  operator,  the  range  officer  sets  each 
pointer  at  the  normal  of  its  set  of  correction  curves. 

The  difference  in  ranges  on  the  scale  graduated  on  the  body  and 
the  movable  scale  under  the  cross  wire  in  excess  of  the  mechanical 
error  will  be  due  to  inaccurate  work  on  the  part  of  the  range-board 
operator. 

Other  methods  of  test  may  he  devised  if  found  desirable. 

CARE  AND  PRESERVATION. 

17.  The  parts  subject  to  friction  should  be  oiled  with  clock  oil 
when  necessary.  An  excess  of  oil  should  not  be  used.  All  parts 
should  be  kept  free  from  dust  and  the  board  kept  covered  when  not 
in  use. 


No. 


Nomenclature. 


Drawing. 


Box 

Chart  board 

Correction  chart 

Canvas  chart  mount 

Eyelets  in  canvas 

Gear  bracket 

Gear  cover 

Chain-adjusting  screw 

Chain  for  ruler  and  counterweight 

Chain  sprocket 

Bar  for  curve  indicators 

Cvtrve  indicator 

Counterweight 

Ruler  clamp  screw 

Sprocket  stand  (right) 

Sprocket  stand  (left) , 

Stationary  bar 

Movable  bar 

Movable  scale 

Body 

Knob 

Pointer 

Movable  clamp 

Range  pointer , 

Reading  glass 

Cap 

Chain  screw 

Eccentric 

Stop  screw 


22-3-2 
22-3-2 
F.  A.  3846 
F.  A. 3847 
2^3-3 
22-3-8 
22-3-S 
22-3-8 
22-3-3 
22-3-3 
22-2-3 
22-3-3 
22-3-9 
22-3-9 
22-3-4 
22-3-4 
22-3-8 
22-3-8 
22-3-8 
22-3-7 
22-3-9 
22-3-8 
22-3-8 
22-3-8 
22-3-9 
22-3-8 
22-3-8 
22-3-8 
22-3-8 


10 


Nomenclature. 


Wing  nut 

Clamp  spring .* 

Ruler  clamp '. 

Guide  plate  (left) 

Guide  plate  (right) 

Reading-glass  holder  support  slide 

Main  gear 

Movable-bar  gear 

Gear  and  pinion 

Idler  gear 

Reading-glass  retaining  screw 

Reading-glass  holder  hinge  screw 

Cross-wire  clip 

Cross-wire  screw 

Cross  wire * 

Reading-glass  holder 

Brass  screw  eye  for  counterweight 

Clamp 

Nut  for  clamp 

Washer  for  clamp 

Bearing  strip  for  ruler  (right),  countersunk  holes : . 

Bearing  strip  for  ruler  (left),  elongated  holes 

Upper  clip 

Lower  clip 

Lower  clip  (straight) 

0.312  (t^)  Bennett  handy  oiler 

Counterweight  rollers 

Clamping  screw  for  curve-indicator  bar 

Axle  for  counterweight  rollers 

Axis  for  chain  sprocket 

Name  plate 

Support  for  curve-indicator  bar 

Guide  strip  for  counterweight  (right) 

Guide  strip  for  counterweight  (left) 

Chain  for  clamping  screw  for  curve-indicator  bar 

Rod  for  canvas  (long) 

Pipe  for  canvas  (long) 

Rod  for  canvas  (short) 

Pipe  for  canvas  (short) 

0.099  by  0.16  headless  steel  screw 

No.  5  by  0.625  flathead  wood  screw 

No.  5  by  0.625  roundhead  wood  screw 

0.177  by  0.5  standard  filister  head  bronze  screw 

No.  6  by  0.875  flathead  wood  screw. . , 

0.216  by  0.375  standard  filister  head  bronze  screw 

0.099  by  0.187  (^)  standard  filister  head  steel  screw 

0.064  by  0.31  bronze  pin 

0.112  by  0.125  standard  roundhead  bronze  screw 

0.093  (i^)  by  0.56  taper  pin 

0.093  (^)  by  0.75  taper  pin 

0.112  by  0.25  standard  countersunk  head  bronze  screws 
0.177  bv  0.75  standard  filister  head  bronze  screws 


11 


LIST  OF  DRAWINGS. 

Assembled  views  and  canvas  chart  mount F.  A.  3847 

Details ,  wood ' 22-3-2 

Details,  metal 22-3-3 

Details,  travel  device 22-3-4 

Correction  ruler,  assembled  views  and  body 22-3-7 

Correction  ruler,  details 22-3-8 

Correction  ruler,  details 22-3-9 

War  Department, 

Office  of  the  Chief  of  Ordnance, 

Washington,  December  SO,  1914. 
Form  No.  1663. 
May  20, 1908. 

Revised  December  30, 1914. 
34082-161. 
Ed.  Aug.  24-17—500. 


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